The demand for less expensive, and yet more reliable integrated circuit components for use in communication, imaging and high-quality video applications continues to increase rapidly. As a result, integrated circuit manufacturers are requiring greater accuracy in voltage references for such components and devices to meet the design requirements of such a myriad of emerging applications.
Voltage references are generally required to provide a substantially constant output voltage despite gradual or momentary changes in input voltage, output current or temperature. In particular, many designers have utilized bandgap reference circuits due to their ability to provide a stable voltage supply that is insensitive to temperature variations over a wide temperature range. These bandgap references rely on certain temperature-dependent characteristics of the base-emitter voltage, V.sub.be, of a transistor. Typically, these bandgap reference circuits operate on the principle of compensating the negative temperature coefficient of a bipolar transistor's base-emitter voltage, V.sub.be, with the positive temperature coefficient of the thermal voltage, i.e., with V.sub.Thermal =kT/q, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, and q is the electronic charge. In general, the negative temperature coefficient of the base-emitter voltage V.sub.be is summed with the positive temperature coefficient of the thermal voltage V.sub.Thermal, which is appropriately scaled such that the resultant summation provides a zero temperature coefficient. One such well-known method is the Brokaw bandgap cell, as shown in FIG. 1.
While the bandgap reference is configured to be independent of temperature, or at least linear with temperature, in practice the bandgap reference will typically produce a reference voltage having a derivative of zero for only one given temperature. This characteristic of the bandgap reference is mainly due to the fact that the V.sub.be (T) term is a non-linear function. In other words, an inherent variation exists for the base-emitter voltage V.sub.be of a transistor with respect to temperature. In particular, the bandgap reference generates a strong second-order term that varies with Tln(T), and which limits the temperature drift performance of such a reference, i.e., causes deviation of the reference voltage with temperature. While these second order terms may be relatively small, their impact can prove highly undesirable for many applications.
Various methods have been used to compensate for the temperature curvature characteristics for bandgap references. These methods have included the addition of circuitry which first attempts to measure the temperature curvature of the base-emitter voltage V.sub.be, and then sum the measured temperature curvature term with the bandgap reference output. Other methods have included the addition of circuitry that approximates the temperature curvature with a squared function of the temperature, such as by utilizing a proportional-to-absolute-temperature PTAT current through a resistor having a given temperature coefficient TC. While these methods may be utilized with some success, limitations exist over process variations or over a wide range of temperature, such as between the range of -50.degree. C. to 150.degree. C. Most notably, many of these methods have been configured to address applications utilizing bipolar transistors, but can not be utilized effectively with CMOS applications. This limitation of prior art methods results in part because CMOS processes typically have one parasitic vertical bipolar transistor whose collector is configured in a manner that limits the use of vertical bipolar transistor to that of an emitter follower, thus limiting the application of such techniques to CMOS processes.
Others have attempted to provide a bandgap reference voltage for CMOS processes through the comparison of MOS source-gate voltages to perform curvature compensation. However, these approximation techniques have not proven to be as successful as required by emerging applications.
Accordingly, as one will appreciate, a need exist for an improved temperature curvature compensation method and circuit for bandgap references, and in particular one that may be utilized effectively in CMOS applications.